Conventionally, a control system that feedback-controls an engine speed during idling of an internal combustion engine has been proposed in Patent Literature 1. This engine includes a bypass passage that bypasses a throttle valve, an idling adjusting valve for opening and closing the bypass passage, an ignition coil, and so forth. As described hereinafter, the control system controls both the intake air amount and the ignition timing during idling of the engine such that the engine speed converges to a target engine speed.
More specifically, first, during idling, the target engine speed, a basic intake air amount, and a basic ignition timing are set according to operating conditions of the engine. Next, the difference between an actual engine speed and the target engine speed is calculated, and the feedback correction amounts of the intake air amount and the ignition timing are calculated based on the difference. Then, a control input to the idling adjusting valve is calculated as a value corresponding to the sum of the basic intake air amount and the feedback correction amount of the intake air amount, and a control input to a spark plug is calculated as a value corresponding to the sum of the basic ignition timing and the feedback correction amount of the ignition timing. Further, the idling adjusting valve and the spark plug are driven by respective drive signals corresponding to the above control inputs for control of both the intake air amount and the ignition timing, whereby the engine speed is feedback-controlled such that it converges to the target engine speed.
[Patent Literature 1] Japanese Laid-Open Patent Publication (Kokai) No. H05-222997
According to the above described conventional control system, although the engine speed is feedback-controlled through two control processes by the two control inputs, there is a fear that the two control processes interfere with each other, since the characteristics of the control processes are different from each other. More specifically, when the ignition timing is controlled, response delay and dead time of the control are short, and hence it is possible to cause the engine speed to quickly converge to the target engine speed. Further, although in the ignition timing control, excellent control accuracy can be ensured due to its high resolution of control (the rate of change in the engine speed with respect to the minimum ignition control input being small), the range of change in the ignition timing is limited from with a view to avoiding degradation of the operating conditions of the engine. For example, in cases where the target engine speed is temporarily and sharply increased e.g. by racing by a driver during idling, if the ignition timing is controlled in a manner coping with the increase in the target engine speed, the retard amount of ignition timing becomes too large, which can result in reduction of combustion efficiency. To avoid this inconvenience, the range of change in the ignition timing is limited.
On the other hand, when the intake air amount is controlled, this control process is lower in resolution of control than the ignition timing control process, and hence it is possible to cope with a large change in the target engine speed. However, the engine speed cannot be controlled by the control process as finely as by the ignition timing control process, which results in degraded control accuracy. Moreover, response delay and dead time are larger in the feedback control process of the intake air amount than in the feedback control process of the ignition timing. This results in the degraded convergence of the engine speed to the target engine speed.
As to the conventional control system, there is a fear that the two control processes interfere with each other due to the above-described differences in characteristics therebetween, causing reduction of both the stability and the accuracy of the control.
Furthermore, in the general feedback control methods, such as PI control, when a sudden change in the target engine speed as mentioned above has occurred, the engine speed control tends to cause overshooting or undershooting of the target engine speed and an oscillating behavior of the controlled engine speed, and attempts to avoid these inconveniences cause reduction of the convergence rate of the engine speed to the target engine speed. In short, it is difficult to positively maintain both the converging behavior and convergence rate of the engine speed to the target engine speed in excellent conditions, which results in further degradation of both the stability and the accuracy of the control.
The present invention has been made to provide a solution to the above-described problems, and an object thereof is to provide a control system which is capable of enhancing both the stability and the accuracy of control when the output of a controlled object is feedback-controlled by a plurality of control inputs.